1. Field of the Invention
The present invention is directed to a method and an apparatus for determining when a partially or completely collapsed lung has been opened and to a use of the apparatus.
2. Description of the Prior Art
In Swedish Application 501 560 corresponding to U.S. Pat. No. 5,575,283, the importance of opening a collapsed lung and keeping it open is described. A ventilator which can calculate an opening pressure for a lung based on measured pressure and volume values is also disclosed. When correlating pressure and volume for a collapsed lung during the opening procedure, an inflection point will normally be present in the curve obtained. This inflection point can be determined mathematically from the obtained curve and represents an opening pressure for the lung.
Determining the inflection point, however, is difficult. The calculated opening pressure will vary depending on the mathematical method used, particularly because the inflection point is not sharp and easy to identify. The reason for this is that the alveoli do not open all at once, but rather in groups forming small clusters or compartments within the lungs. Therefore, the opening of small compartments of alveoli at different pressures will result in several smaller inflection points in the pressure-volume curve. Some of these compartments (or alveoli) can require a higher pressure in order to be opened. When such compartments are present and treatment is given based on the calculated opening pressure, such compartments of the lungs remain closed during the entire respiratory cycle, resulting in an insufficient, or at least impeded, gas exchange for the patient.
The pressure-volume curve analysis described above, therefore, does not indicate when the lung is opened. There is no known apparatus which can determine when the lung is opened or at which pressure the lung is opened.